Despite significant improvement in many areas of parachute technology since the introduction of the parachute, injuries to jumpers and damage to cargo upon landing are still problems today.
Parachutes in general, and commercial sport parachutes in particular, have evolved over the years into highly sophisticated systems, and now often include features that improve the safety, maneuverability, and overall reliability of the parachutes. In certain applications, military parachutes have followed the general evolution of sport parachute systems. For example, the U.S. Marine Corps has recently begun using a highly maneuverable square ram-air parachute system for introduction of reconnaissance and special operations forces.
On the other hand, the U.S. Army's standard, round-canopy T-10 parachute used by combat paratroopers has been in service for nearly 50 years without major changes. The Army has identified excessive rate of descent as the chief contributor to paratrooper injuries. In fact, rate of descent and the resulting force of impact at touchdown has increased dramatically in recent years, primarily because the parachutists are carrying heavier combat equipment loads. The T-10 parachute was originally designed for a 250-lb. paratrooper, but with present-day equipment requirements, the T-10 parachute is being used for total suspended weights of up to 380 lbs. The added weight means that paratroopers descend faster and strike the ground with substantially greater force, making them more prone to injury.
Superficially, the solution to excessive rate of descent seems simple enough: use a larger parachute and/or a higher drag parachute. However, the existing T-10 combat parachute is already bulky, making it uncomfortable and awkward to wear and making exit from the airplane more difficult. A larger parachute would tend to add weight and bulk, thus making it even less comfortable and adding to soldier fatigue and degrading mobility. In addition, the slower rate of descent keeps the paratrooper in the air longer, thus increasing his vulnerability to enemy fire during a drop. A slower rate of descent can also make landing on target much more difficult in windy conditions, and will likely increase the dispersion of troops over the landing zone.
Cargo drop systems have similar problems. Although not as critical as with personnel parachutes, size, and particularly weight, are always important considerations for equipment that must be carried aloft. Within the parameters of any transport aircraft's lift capacity, every additional pound of parachute equipment that must be carried reduces the delivered cargo load correspondingly. Also, windy conditions are more of a problem for a cargo drop than for a paratrooper, because the cargo has virtually no directional control.
Therefore, the ideal parachute system for most military applications would have a relatively high rate of descent to minimize "hang time," and a low impact velocity to minimize landing injuries or cargo damage.
Several attempts have been made over the years to mitigate landing velocity problems in both cargo drop and personal parachute systems. Systems used have included air cushions, energy absorbing struts, and retro-rockets, as well as various approaches for reducing the distance between load and parachute just prior to ground impact.
One of the earliest systems for reducing the distance between the load and the parachute is disclosed in U.S. Pat. No. 2,386,395 to Hart, which utilizes a framework containing a plurality of elastic bands to store energy. The bands are either pre-loaded, or use the opening shock of the parachute to cock the system. Just prior to touchdown, the cocked system is released and pulls the jumper (or load) towards the parachute.
U.S. Pat. No. 2,477,907 to Smith employs a cable drum to effect a reduction in the distance between the payload and the parachute. Just prior to touchdown, a gas cartridge drives the drum to reel in the cable connecting the payload to the parachute.
U.S. Pat. No. 2,492,501 to Robins discloses a cargo drop method for arresting landing velocity. The load support line is retracted using a multi-reeled pulley system driven by a gas powered cylinder.
U.S. Pat. No. 3,109,615 to Fritz uses a ballistic cable reel to reduce impact in cargo drops. U.S. Pat. No. 3,387,805 to Barnett is a complex parachute system with various features aimed at reducing opening shock, improving canopy reliability, and including soft landing features. As a part of its soft landing apparatus, the system uses a cable drum to foreshorten the distance between payload and parachute just prior to touchdown.
U.S. Pat. No. 4,127,246 to Andres uses elastic cords to reduce landing velocity. A steel cable maintains a fixed distance between the load and the parachute. At the proper distance from touchdown, a probe or other sensing means fires a cable cutter, releasing the load. Free fall of the load stretches the elastic cords until the load rebounds relative to the parachute, reducing the actual rate of descent at impact.
The U.S. Army has also conducted analysis and experiments on parachute systems that reduce landing velocity by accelerating the load toward the parachute just prior to impact. A report by the Natick Research, Development and Engineering Center Report TR-97/014, entitled "Predictive Model of a Parachute Retraction Soft Landing System," describes one such effort. The report describes a computational model for system analysis, as well as hardware for experimental air drop tests. The apparatus used for these tests consisted of a multi-reeled pulley system driven by a piston/cylinder assembly, somewhat similar to the system disclosed in U.S. Pat. 2,492,501, to Robins, cited above.